1. Field of the Present Invention
The present invention relates generally to a magnetic disk drive ("disk drive"), and more particularly to a unique servo pattern for micro-jogging a magnetoresistive transducer head during a read operation.
2. Description of the Related Art
A conventional disk drive has a head disk assembly ("HDA") including at least one magnetic disk ("disk"), a spindle motor for rapidly rotating the disk, and a head stack assembly ("HSA") that includes a transducer head for reading and writing data. The transducer head is controllably positioned by a servo system in order to read or write information from or to particular tracks on the disk. The typical HSA has two primary portions: (1) An actuator assembly that moves in response to the servo control system and (2) A head gimbal assembly ("HGA") that extends from the actuator assembly and biases the head toward the disk. The typical HSA further includes a flex circuit on the side of the actuator body and electrical conductors which run between the flex circuit and the head to transmit read and write signals to and from the head.
The industry presently prefers a "rotary" or "swing-type" actuator assembly which conventionally comprises an actuator body that rotates on a pivot assembly between limited positions, a coil that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor, and an actuator arm that extends from the opposite side of the actuator body to support the HGA.
Each surface of each disk conventionally contains a plurality of concentric tracks of servo data segments and user data segments that are alternately located around each track to define a corresponding plurality of servo wedges and corresponding data wedges. The servo data comprises track ID fields and servo bursts (an alternating pattern of magnetic transitions) which are used by the servo system to align the transducer head with a particular data track. The servo control system moves the transducer head toward a desired track during a "seek" mode based on the track ID field. Once the transducer head is over the desired track, the servo control system enters a "track follow" mode and uses the servo bursts to keep the transducer head over the data.
For many years, the industry used inductive heads where the same transducer is used for reading and writing. More recently, however, the industry has begun using magnetoresistive transducers which are only capable of reading. Therefore, two separate heads--an inductive head for writing and a magnetoresistive head for reading--are required. The separate read and write heads are necessarily spaced one behind the other. A variable skew between the two heads is introduced by this spacing due to the tangential relationship of the transducers to a circular data track on the disk as they are positioned over the disk from inner to outer tracks. The relative positions of the two heads may be set during manufacturing process to bias the skew so that the heads may be, for example, aligned when positioned over the innermost tracks and skewed when positioned over the outermost tracks or other arrangements suitable for a particular design objective. Manufacturing tolerances can also cause a small shift in the relative centerlines of the heads with respect to one another.
A problem exists with using the magnetoresistive transducer head, therefore, because the servo bursts are read with the magnetoresistive read head but the data tracks are written with the inductive write head that is skewed or variably displaced from the read head depending on the radial position over the disk. As a result of this physical displacement between the two heads, it is necessary to offset or "micro-jog" the transducer head during the read operation or during the write operation.
The servo control system usually aligns the read head with the adjacent edges of a servo burst pair while writing, rather than reading so that the data is written while track following on the equal signal "center line" of the servo burst pair. For reference in this disclosure, the equal signal center line may be termed a "burst pair center line" of a selected pair of servo bursts. The read head converts the amplitude of each burst to an electrical signal which will be used by a disk microprocessor to determine the required control effort or correction needed to track follow. In such case, the center line of the data track may be displaced from the burst pair center line of the servo burst pair by the physical displacement between the read and write heads at that particular radius. Later, when reading the data, the servo control system micro-jogs the read head away from the burst pair center line of the two bursts in the same direction as the original physical displacement so that the read head passes over the center line of the data track in order to maximize the amplitude of data signals read from the disk thereby increasing the signal-to-noise ratio.
A continuing problem is that the magnetoresistive read head is narrow compared to the write head, and therefore has a limited range of linearity across a full track as it reads a sequence of bursts to provide a position error signal. In other words, as the read head position is displaced from the center line of a servo burst pair, there is a relatively small amount of displacement over which the signal produced by the servo bursts remains linearly related. In the inventors' experience, the magnetoresistive read head can only be displaced from the center line of a servo burst pair by about 1/6 or 16.67 percent of a track pitch (the distance between center lines of adjacent tracks) and still remain within a useable range of linearity.
Depending on the relative size of the read and write heads, their longitudinal position, and their lateral position, however, the skew or relative displacement between the read and write transducers may vary by more than 16.67 percent of a track pitch as the swing-type actuator moves the heads from the inside diameter to the outside diameter of the disk. If we adopt a convention wherein we speak of the write head's displacement relative to the read head, and wherein a positive displacement means the write head is located farther to an outside diameter than the read head, the inventors have found that the displacement range can be as large as -45 percent to +45 percent of a track pitch as the actuator moves the transducer from the inside diameter to the outside diameter.
One prior patent attempting to address the narrow region of linearity available when micro-jogging a magnetoresistive transducer is U.S. Pat. No. 5,587,850 (the '850 patent) issued to Luan Ton--that on Dec. 24, 1996, and assigned to Quantum Corporation. The '850 patent generally teaches a servo sector arrangement where a group of servo burst patterns which are each 2/3 of a track pitch wide have centers which are offset from each other by one-third of a track pitch in a "quadrature" fashion where each successive burst overlaps a previous burst by one-half of a burst width. By offsetting the bursts from a track identification field by a "micro-jog" distance, the read head can track on the centerline of a burst pair while the write head is positioned at the data track centerline. Necessarily during read operations, the servo must track follow at a point which is offset from the burst pair centerline and is therefore dependent on operating within a linear response range. This will be limited in the case presented by the '850 patent, as indicated in column 8 lines 55-57, to 16% of the track pitch. In current disk drives having even higher track density, the linear response range may be in the range of 32% (.+-.16%) of a track pitch. As indicated above however, the relative displacement of the read and write heads to the track centerline can vary from -45% to +45% of track pitch. Therefore the servo pattern of the '850 patent would not provide sufficient linear response range. Additionally, as indicated above, the burst width suggested by the '850 patent is on the order of 2/3 of a track pitch. This generally means that the servo writer must make more passes to write the narrow 2/3 track pitch servo bursts compared to conventional servo burst patterns which are as wide as the track pitch, even though the servo writing process is already considered a bottleneck in manufacturing disk drives.
Accordingly, there is a need for a disk drive with a servo burst pattern which provides extended linear response for micro-jogging regions over selected portions of the disk without an increase in servo writing time during manufacture to accommodate narrow width servo bursts.